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LES, GREAT BRITAIN.

SING} BEE'ICAIJE AND METH'OD Oil MAKENG SAT/IE.

No Drawing. Application. filed June 20, 1927, Serial No. 202,756, and in Great Britain July 7, 1923.

This invention relates to improved rolls for rolling metals.

Quenched rolls have the disadvanta 'e that enormous internal stresses are generated on quenching, while hair-crac us and similar surface flaws are liz ble to develop in the worrring surface, these defects causing snails during; the working oi? the roll. In chilled cast iron rolls, which brittle and have poor n1e chanical strength throne out, the chilled iron rim or chill has a rate of expansion under heat which is twice that of the grey iron core, so that enormous internal stresses set up on increase oi? temperature. Great internal stresses are also set up when the roll is east, owing to the rapid cooling; or": the roll body, and, moreover, when the chill is worn out it cannot be renewed. A "further common disadvantage of quenched and chilled rolls is the diiiiculty in manufacturing them for rolling complicated sections.

The object of this invention is to obviate these disadvantages and to provide rolls for rolling: metals having higher physical propert-ies than the rolls at present in general use and which may be manufactured in complicated shapes.

According to the present invention 1 provide a roll for rolling meta-ls, made in case hardened alloy steel and having its case-hardened portion oi a hard martensitic troostitic, cementitic or allied. structure and remaining); portion of a soft pearlitic or allied structure, both structu. s being obtained with the same slow rate of cool ing from high teimaeratures, that is to say, such rate of coolin as may be obtained without external aids, such as immersion in, or sprayin,q with, any liquid. To this end, certain e ements such 'l'c-r Oll'iilllPlG, as nicltel or mar ,uese, are introduced into the steel, an d. which, i u the IVWSCHCO oi suiiicient carbon, tend. to retain the m l. in the hard martensitic, troostitic. cement-die or allied condition, after a rate of cooling so slow that a steel of the same carbon content, but without said elen'ients, would have passed into the relatively soft pearlitic condition.

In these alloys, the eiiFec-t of carbon on this tendencyfior a given proportion. of the special element, or elements is cumulative, that is, a suitably chos alloy will pearlitic, soft and tough, with low percentage oi carbon, but treostitic, marteusitic, cementitic or oi make the roll Oil an alloy containing a low percentage oi carbon and such amounts of the other constituents will give, on slow cooling, with this low percentage of carbon, at pearlitie or allied structure, and with a suit-- able high percentage of carbon, to be introduced into the case by case-carburizing a martensitic, troostitic, cementitic or allied structure, as may be required, thus attaining maximum hardness for the case without the necessity for reducing the temperature of the roll from high temperatures in water or other liquid, with all the risks attendant on this procedure.

(it the elements which may be usefully employed :tor alloying; in the process, manganese, vanadium, tungsten, chromium and molybdenum, favour the absorption of carbon by solid steel duringcase-hardeni .g, while nickel tends to hinder it where solid casehardening cements are used. Thus, if nickel is used in conjunction with a solid cement, one or more of the first mentioned elements should be used with it to counteract this tendency, while this addition of a further special element or elements. can generally also by careful choice be made to enhance the p'eneral qualities of the resultant alloy. Nickel and chromium, for ex: mple, form a very suitable combination for many purposes.

lly way oi? example, I will. describe the mail-ting; of a roll, for cold rolling of metals, in a nickel-chrome steel alloy. For hardness without unnecessary brittleness, the case should be at, or slightly above, the eutectoid composition, say, 1.0 per cent carbon. For normal cooling in air, with 1.0 per cent carbon, 5.0 per cent nickel will give a martensitie structure, but with carbon contents appreciably below 0.80 per cent the structure will be pearlitic. For a nickel content Oil 5.0 cent 0.80 per cent of chromium will be sullicient to ensure carbon absorbing propertics with solid cements, and will give a harder case without unduly increasing brittleness while, further, the structure will not coarsen under heat treatment in the presence of so much nickel.

I cast or forge the roll in a steel containing approximately Ni. Cr. C 5.0% 0.80% 0.20%

The roll is then well annealed and is allowed to cool to room temperature and then machined, the body being brought to a smooth lini sh and left, say, .015 overall full to final size. After machining, the roll is case carburized. in a box having removable ends, or other device, to allow of its rapid removal whilst hot.

\Vith the carburizing mixture consisting of 60 per cent oak, or beech, charcoal and 10 per cent barium carbonate, a temperature of 900 C. will give approximately 1.0 per cent carbon in the case for this composition, the time taken obviously varying with the depth of case required and being easily determined. (l Vhere very deep cases are required, it is preferable to use pure charcoal at higher temperatures, reducing the nickel and the chromium contents to correspond with the higher carbon percentage thus obtained in the case.) At the conclusion of the carburizing proce dure, the grain structure of the core may be refined by allowing the roll to cool down in its carburizing box to well below the thermal critical range of the core material, whereafter it is reheated to approximately 100 C. above said range, and after being kept at this temperature for a sufiicient time to ensure that the structure shall have passed into the solid solution condition, the roll in its box is removed from the furnace. On removal from the furnace, the roll is stripped, and allowed to cool in air. Where an exceptionally tough core is required the refining process may be doubled, that is to say, the roll after its preliminary cooling from the case-hardening temperature may be first reheated up to, and furnace cooled from, above the critical range of the core, then re-heated up to ust below the said critical range and cooled in air.

When no grain structure refining procedure is utilized, the roll will be removed from the furnace at the lowest ten'iperature from which air cooling will give the required degree of hardness in the case.

After cooling, the roll is finally ground to size and polished, the machining allowance given above being sufficient for the re moval of any slight decarburized bark (that is, the extremely thin skin of metal,

which is robbed of its carbon by oxidation during the final cooling down in air).

It should be noted that when the final cooling of the roll in air is for any reason undesirable, or when either extreme hardness on the working face of the roll is unnecessary or a composition may be used which, with the amount of carbon to be introduced by case-hardening in the particular instance, Will be so far removed from the structuretransition range as to ensure that, even on very slow cooling, the case structure will remain in the required hard condition, the final cooling of the roll, may take place in the carburizing box either in or out of the furnace and not in air. In this case, the case-hardening material packed around the roll in the carburizing box is not disturbed.

I claim:

1. A roll for rolling metals, made in casehardened alloy steel and having its case-hardened portion of a hard structure, and the remaining portion of a soft pearlitic or allied structure, both structures being obtained with the same slow rate of cooling of the roll from high temperatures.

2. A roll as claimed in claim 1, containing one or more of the elements manganese, vanadium, tungsten, chromium, or molybdenum as constituents in the steel alloy.

3. A roll as claimed in claim 1, containing the element nickel as a constituent in the steel alloy, and one or more of the elements manganese, vanadium, tungsten, chromium or molybdenum.

4. A roll as claimed in claim 1 having an interior with a low carbon content and having a high percentage of carbon in the case.

5. A method of making a roll for rolling metals, which consists in providing an alloy steel containing approximately Ni. Cr. C 5.0% 0.80% 0.20%

forming a roll therefrom, annealing the product, cooling it, machining the cooled product, case-hardening it, allowing it to cool to below the thermal critical range of the core, reheating to about 100 C. above said range keeping it at this temperature for a sufficient time to ensure that the structure shall have passed into the solid solution condition, and allowing the roll to cool.

6. A method of making a roll for rolling metals, which has the steps of subjecting an alloy steel to casehardening in the presence of a high temperature, and then cooling at a slow rate both the case and the interior, such slow rate of cooling being the same for both and a rate that produces a hard case and a soft core.

7. A method of making a roll for rolling metals, which has the steps of providing an alloy steel with a low percentage of carbon, and with a constituent that on slow cooling from a high temperature will give a soft, pearlitic or allied structure, increasing the carbon content by the application of a high temperature, and then cooling at a slow rate both the carhurized exterior, such rate being one that produces a hard case and a soft core.

8. A method of making a roll for rolling metals, which has the steps of subjecting an alloy steel to case-hardening in the presence of a high temperature, and then cooling at a slow rate both the case and the interior, such slow rate of cooling being the same for both and a rate that produces a 10 hard case and a soft core, reheating the roll to the critical range of the core, furnacecooling it, again reheating short of said critical range, and then cooling.

In testimony whereof I have signed this 15 specification.

FRANCIS DAVID CORBIN. 

